1. Field of the Invention
This invention relates to a lighting lamp excellent in color rendering properties in a red region, and specifically to a lighting lamp suitable for use in a light source for the projection of an image formed on a color liquid crystal panel, and the like.
2. Description of the Related Art
Short-arc metal halide lamps in which a rare earth metal and a halogen are sealed are used as light sources for studio-lighting and the like because they are high in luminous efficiency in a wavelength region of visible rays and excellent in color rendering properties owing to their possession of spectral characteristics similar to those of sunlight.
However, light sources for the projection of images formed on color liquid crystal panels require spectral characteristics different from those of sunlight and fit for the sensitivity characteristics of color filters thereof. Therefore, conventional short-arc metal halide lamps have been accompanied by a problem that the radiant intensity in a red region is relatively low and the color rendering properties of color liquid crystal displays are hence deteriorated.
In order to enhance the radiant intensity in the red region on the other hand, it is effective to seal lithium within a light-emitting tube.
However, the sealing of lithium involves the following problem. Since lithium is high in reactivity with the glass material making up the light-emitting tube, devitrification of the light-emitting tube, occurs during its operation, so that the luminous flux radiated is reduced as a whole and the radiant intensity of red light is rapidly decreased. Therefore, such sealing fails to provide a sufficient service life.
With the foregoing in view, the present inventor has carried out an extensive investigation. As a result, it has been found that when lutetium (Lu) is sealed within a light-emitting tube, lithium (Li) is additionally sealed in a specific proportion to the lutetium and a halogen is further sealed in a specific range in excess of the lutetium and the lithium, the devitrification of the light-emitting tube can be avoided to diminish the attenuation of luminous flux and spectral characteristics high in radiant intensity of red light are obtained.